PROJECT SUMMARY/ABSTRACT Disturbed sleep, and especially insomnia and obstructive sleep apnea (OSA), is highly prevalent and associated with increased risk for elevated blood pressure (BP) and cardiovascular disease. Unfortunately, despite the substantial public health burden of disturbed sleep, standard treatments are often limited by poor adherence, inadequate availability, and/or significant side effects. As such, identification of alternative approaches to mitigate disturbed sleep is greatly needed. In contrast to increasing engagement in exercise, we propose that reducing sedentary behavior (SED), or time spent sitting, is a novel and feasible approach to reduce sleep disturbance. Preliminary evidence from our group indicates that greater SED is associated with worse sleep quality and greater OSA severity, and our pilot data suggest that short-term SED reduction improves sleep continuity (i.e., reduced wakefulness after sleep onset [WASO]). Given our group's work demonstrating the impact of disturbed sleep on behavior modification and the relationship between disturbed sleep and BP, we also propose that disturbed sleep at baseline could blunt the adherence to attempted SED reduction and impact its cardiovascular health benefits. Therefore, the goal of this ancillary study application is to test the hypotheses that SED reduction will improve sleep and that the presence of disturbed sleep at baseline will reduce the effectiveness of SED reduction efforts by adding comprehensive sleep assessments to an ongoing randomized clinical trial that is examining the effect of SED reduction on BP. In this parent trial, 300 desk workers with elevated BP are randomized to a 3-month multicomponent behavioral intervention aimed at replacing 2-4 hr/day of SED with light-intensity activity or a 3-month no-contact control condition. In the remaining sample of the parent trial (estimated to be N=210 by the start of the ancillary study), we will assess sleep at baseline and post-intervention using 7 nights of wrist-worn actigraphy and 1 night of home-based polysomnography (PSG), yielding objective measures of sleep quality (WASO), total sleep time (TST), OSA severity (apnea-hypopnea index [AHI]), and sleep depth (slow-wave sleep [SWS]). We will evaluate the effect of the SED reduction intervention on actigraphy-assessed WASO (primary outcome) and TST and PSG-assessed AHI and SWS (Aim 1). We will also examine the effect of disturbed sleep at baseline on subsequent SED reduction and BP improvement (Aim 2). Results from this ancillary study, which efficiently leverages an ongoing and well-functioning clinical trial, will have a significant impact on behavioral sleep medicine recommendations, as it will provide new insight into whether SED reduction is a viable approach for reducing sleep disturbance. Moreover, these findings will inform the continuing development of SED guidelines and help optimize future SED reduction interventions by evaluating whether sleep disturbance impacts SED reduction efforts.